The present invention relates to an electrostrictive polymeric material, articles including such polymers, and a method for manufacturing the polymeric material and articles. In particular, the electrostrictive polymeric material includes a random copolymer of vinylidene fluoride and hexafluoropropylene.
Most polymers have electrostrictive properties to a varying degree, such that they exhibit an electric field-induced strain response. These materials undergo an elastic deformation, or strain, when exposed to an electric field, with the strain being proportional to the square of the applied field. Conversely, they can produce an electrical voltage when the electrostrictive material is subject to a deformation while in an electric field. For example, U.S. Pat. No. 4,298,719 discloses various double oriented polyvinylidene fluoride polymers that are excellent as dielectrics and reports that they have various properties including being electrostrictive.
Certain polymeric materials are known to exhibit fairly high electric field-induced strain responses. Materials exhibiting large electrostrictive strains were first observed in a polyurethane elastomer. The polyurethane elastomer exhibited strains greater than 3 percent when subjected to electric fields of up to 40 MV/m [Ma, Z. et al., High Field Electrostrictive Response of Polymers, Journal of Polymer Science: Part B, Polymer Physics, vol. 32, 2721-2731 (1994)]. An elastic modulus of about 0.01 Gpa were observed for these polyurethane elastomers Similarly, electrostrictive strains of about 4 percent have been observed under electric fields of 150 MV/m for a poly(vinylidene fluoride-trifluoroethylene) copolymer that was melt pressed, cooled stretched, and then irradiated with a high energy electron beam [Zhang, Q. M., et al., Giant Electrostriction and Relaxor Ferroelectric Behavior in Electron-Irradiated Poly(vinylidene-fluoride trifluoroethylene) Copolymer, Science, vol. 280, 2101-2104 (1998)]. The irradiated poly(vinylidene-fluoride trifluoroethylene) copolymer had an elastic modulus of approximately 0.4 Gpa.
Electrostrictive materials that exhibit high field-induced strain responses have applications in transducer, sensor, and actuator technologies; robotics; artificial muscles; and microelectrical mechanical systems (MEMS). There is, however, a need for new and improved materials that exhibit high electrostrictive strains and have improved properties, as well as methods of preparing such polymers and articles including such polymers. The present invention provides such material, processes, and articles.
The present invention is directed to an electrostrictive material that includes a random copolymer of vinylidene fluoride and hexafluoropropylene, wherein the material has a strain response of more than about 2 percent when subjected to an electrical field of at least about 30 megavolts per meter. The electrostrictive material may preferably have a strain response of more than about 3 percent when subjected to an electrical field of at least about 35 megavolts per meter or, more preferably, more than about 4 percent when subjected to an electrical field of at least about 50 megavolts per meter.
The electrostrictive material of the invention includes a copolymer of vinylidene fluoride and hexafluoropropylene and contains from about 75 to 99 mole percent vinylidene fluoride and about 25 to 1 mole percent hexafluoropropylene. In another embodiment the electrostrictive material may contain about 85 to 99 mole percent vinylidene fluoride and about 15 to 1 mole percent hexafluoropropylene or, in another embodiment, from about 90 to 99 mole percent vinylidene fluoride and about 10 to 1 mole percent hexafluoropropylene. In one preferred embodiment, the electrostrictive material contains about 95 percent vinylidene fluoride and about 5 percent hexafluoropropylene.
The electrostrictive material of the invention has a degree of crystallinity that is preferably less than about 60 percent. The Young""s modulus of the electrostrictive material may be above about 0.3 Gpa, above about 0.5 Gpa, or above about 1 Gpa in various embodiments. The electrostrictive material may have a thickness strain constant, dt, greater than about 500 pm/V, greater than about 1000 pm/V, or greater than about 1500 pm/V in various embodiments. The strain energy density of the electrostrictive material may be greater than about 0.1 J/cm3, greater than about 0.2 J/cm3, or greater than about 0.5 J/cm3 in various embodiments.
In another embodiment the electrostrictive material includes a random copolymer of vinylidene fluoride and hexafluoropropylene, wherein the copolymer includes from about 85 to 99 mole percent vinylidene fluoride and from about 15 to 1 mole percent hexafluoropropylene, the material has a strain energy density greater than about 0.2, an elastic modulus greater than about 0.5, and a strain response of more than about 2 percent when subjected to an electrical field of about 30 megavolts per meter. In yet another embodiment, the electrostrictive material includes a random copolymer of vinylidene fluoride and hexafluoropropylene, wherein the copolymer includes from about 90 to 99 mole percent vinylidene fluoride and from about 10 to 1 mole percent hexafluoropropylene, the material has a strain energy density greater than about 0.8, an elastic modulus greater than about 1, and a strain response of more than about 4 percent when subjected to an electrical field of about 50 megavolts per meter.
In another embodiment of the invention, the electrostrictive material includes a random copolymer of vinylidene fluoride and hexafluoropropylene, wherein the material has a strain response of more than about 4 percent under an electrical field of between about 50 to 140 megavolts per meter.
The present invention also relates to a process for making an electrostrictive polymer and articles such as a film. The invention also relates to composite articles that include two or more films. In one embodiment the composite article includes two or more films, prepared according to the invention, wherein each film has an electrode attached to opposing sides of the films and the films are stacked on top of one another. In another embodiment of the composite two or more films, each film having an electrode attached to opposing sides of the film, are adhered to opposing sides of a semi-rigid sheet of material.
The process for forming a film includes the steps of melting a random copolymer of vinylidene fluoride and hexafluoropropylene to form a fluidized polymer; applying sufficient pressure to the fluidized polymer to remove air and form a film; releasing the pressure on the film; and cooling the film sufficiently rapidly to provide a desired strain response and a desired elastic modulus. The random copolymer may contain from about 75 to 99 mole percent vinylidene fluoride and from about 15 to 1 mole percent hexafluoropropylene. The polymer may be melted by heating the polymer to a temperature about 20xc2x0 C. above its melting point and a pressure of at least about 1000 psi.
The film may be rapidly cooled by quenching it in an ice water bath, or cooled by quenching it at room temperature, or cooled at a rate of about 2xc2x0 C. per minute.